I honestly can't see it ever happening in our lifetime that the parks would cease to exist. As others have said, someone else buying it might happen. The thing is, what would change? Who knows really. Sports owners like to change the name of the stadiums to what the company's name is often. But would Disney World ever be renamed? Would someone be that stupid? Egotistical? Could there be a Pepsi World? Scary to think. I'd like to think the "Disney" name is so entrenched into our mindset that people just wouldn't flock to a park named after a soft drink, or whatever.
The thing is, like others have said, if they keep going the way of "corporate" and start ripping out the heart of the parks and going for the attention deficit crowd it will start to look less and less like a park Walt had anything to do with.